A variety of screw and plate type fastener systems have been designed for securing successive layers of roof covering materials (e.g., a water impervious membrane overlying a layer of insulation) to rooftops made of steel, gypsum, tectum, or wood. Such systems generally include an elongated screw which penetrates a flat plate (washer). The plate (sometimes referred to as a stress plate) clamps down the roof covering membrane and/or insulation when the screw is tightened in the rooftop and tends to prevent the membrane or insulation from pulling vertically over the head of the screw.
Screw and plate fasteners are most frequently used on commercial buildings having a flat rooftop. In general, a layer of insulation is laid out overlying the roof deck and fastened down with screw and plate fasteners. A sheet of membrane, typically marketed in rolls, is then laid out over the insulation. Fasteners are then installed along the edges of the membrane sheet, the distance from the edge and the distance between fasteners being determined in accordance with the deck and membrane material and anticipated conditions. More specifically, a minimum force which will cause the membrane to tear away from the fastener (or the fastener to pull out of the roof), is prescribed. The number of fasteners per unit linear distance is chosen to ensure that the membrane and/or insulation will be retained.
To install the respective fastener, a pilot hole is, if needed, drilled at the desired location through the roof membrane, insulation, and deck. A plastic or metal screw is placed through a retaining stress plate and then installed in the hole, engaging the roof deck so that the plate is held tightly against the membrane. After the fasteners have been installed along the edge of the first sheet, a successive sheet of membrane is laid out, with one edge overlapping the edge of the first sheet of membrane covering the fasteners. That edge of the successive sheet is bonded (e.g., chemically or by heating) to the preceding sheet. The other edge of the succeeding sheet is fixed to the roof deck by fasteners in the manner previously described. Thus, the roof is covered by overlapping sheets of membrane. The membrane edges along the edges of the roof are fixed by battens or other conventional techniques.
Problems have been encountered with conventional screw and plate fasteners when employed in such rooftop environments. Wind blowing over the membrane tends to create a negative air pressure, which in turn tends to cause the membrane to pull laterally out from the fastener. To militate against the membrane tearing out from under the anchor due to such lateral forces, downwardly directed cleats, lugs, spikes, ribs or other protrusions on the underside of the stress plate, engage the membrane when the screw is tightened in the rooftop. See, for example, Murphy U.S. Pat. No. 4,787,188, issued Nov. 29, 1988 and Reinwall et al. U.S. Pat. No. 4,726,164 issued Feb. 23, 1989. However, over time the screw tends to cease to provide tension relative to the membrane (force holding the plate against the membrane) at the level originally installed. This may happen because the underlying insulation deteriorates and shrinks due to, e.g., harsh weather conditions, wind vibrations and/or because vibrations from within the building cause the screw to back out (unscrew) from the rooftop. These conditions can cause the head of the screw to pop up, i.e., protrude from the surrounding roofing material, which in turn leads to damage to the underlying membrane, insulation and roof. Loss of preload tension can also cause the lugs on the underside of the stress plate to lose engagement with the membrane, making the membrane more susceptible to pull out due to lateral forces.
Various combinations of metal screws with plastic plates have been proposed for use as roofing anchors. See, for example, DeCaro U.S. Pat. No. 4,361,997 issued Dec. 7, 1982, Hartman U.S. Pat. No. 4,780,039, issued Oct. 25, 1988, Dewey U.S. Pat. Nos. 4,380,413 and 4,545,270, issued Apr. 19, 1983 and Oct. 8, 1985, respectively and Hasan U.S. Pat. Nos. 4,663,910, issued May 12, 1987, 4,712,959, issued Dec. 15, 1987, and 4,757,661, issued July 19, 1988. However, metal screws present an additional problem; they tend to corrode when used in metal decks.
A number of mechanical systems have been proposed for preventing separation of the screw from the plate in such fasteners. Back out can be prevented by preventing the screw from turning (in a reverse direction) relative to the roof. This could be accomplished with a broad headed screw having lugs, spikes, ribs or the like on the underside of the head to engage the membrane and prevent counter-rotation. However, rotation of the lugs, etc., during installation would tend to tear or otherwise damage the membrane. Accordingly, fastener systems have been proposed including a plate with anti-rotation structure which engages the membrane, e.g., spikes (which also militate against lateral pull out of the membrane), a separate screw, and a mechanism to prevent counter-rotation of the screw relative to the plate. Such a system is described in the aforementioned Dewey U.S. Pat. No. 4,380,413. Projecting pawls on the head of a screw, cooperate with projections on a plate as a ratchet system to prevent rotation to effect installation but prevent counter-rotation between the plate and screw. Projecting structures on the underside of the plate engage the roofing material to prevent rotation of the plate relative to the roof.
A similar system employing a ratchet mechanism to prevent the screw from backing out is also described in Giannuzzi U.S. Pat. No. 4,763,456, issued Aug. 16, 1988. However, these various ratchet structures tend to give the fastener assembly an undesirably high profile, and may be susceptible to disengagement due to loss of tension, or failure due to breakage of the ratchet members, due to, e.g., overtightening or undertightening during installation. Other systems employ a threaded connection between the plate and fastener. For example, the aforementioned DeCaro U.S. Pat. No. 4,361,997, describes a fastener with upper and lower sets of threads with an intervening unthreaded area which cooperates with a stress plate bearing anti-rotation structures on its underside. The lower set of threads are threaded through the plate at the job site with the use of a special tool prior to installation. The upper threads engage the plate after the screw is substantially turned into the roofing deck. The anti-rotation structures engage the roof membrane and prevent the plate from turning.
Other systems have been proposed employing a cap over the head of the fastener (see, e.g., Verble U.S. Pat. No. 4,658,558, issued Apr. 12, 1987; Frankovitch U.S. Pat. No. 4,520,606, issued June 4, 1985; Beneze U.S. Pat. No. 4,620,402, issued Nov. 4, 1986) or resilient spring mechanisms to maintain tension (see, e.g., Hewison U.S. Pat. No. 4,616,455, issued Oct. 14, 1986). Application of a bonding or sealing agent over the head of a fastener, between a stress plate and the membrane, or both has also been proposed. See Sandquist U.S. Pat. No. 4,074,501, issued Feb. 21, 1978, and Frankovitch U.S. Pat. Nos. 4,455,804 and 4,467,581, issued June 26, 1984 and Aug. 28, 1984, respectively. Still other systems rely on a nut or similar element disposed on the lower end of the screw beneath the rooftop to hold the fastener in place. See Sargent U.S. Pat. No. 4,727,699, issued Mar. 1, 1988. These fasteners are only partly effective in preventing the fastener from backing out, and require additional structure for that purpose.
Roofing fastener systems with provisions for preventing the head of the screw from protruding beyond the top of the plate, e.g., in the event of loss of installation tension, have also been proposed. For example, such a system, wherein the washer includes a flexible ring about the aperture that receives the screw, is described in Dewey U.S. Pat. No. 4,380,413. Another such system employing a plastic washer having a resilient rib which engages the screw head to hold it down is described in the aforementioned Hasan U.S. Pat. Nos. 4,712,959 and 4,757,661.
A need remains, however, for a roof fastening system that provides a high pull out value, simplicity in structure, and which can be easily and simply installed without requiring special tools.
Friction welding techniques, such as spin-welding, are generally known and have been used to secure plastic members together. In particular, friction welding has been proposed as a means for securing construction fasteners. See Cearlock et al. U.S. Pat. No. 4,477,307, issued Oct. 16, 1984, Pliml, Jr. U.S. Pat. No. 4,752,171, issued June 21, 1988, Crossman et al. U.S. Pat. No. 4,716,699, issued Jan. 5, 1988, and Nelson U.S. Pat. No. 4,568,215 issued Feb. 4, 1986.
In general, precoating of fastening devices with non-blocking, solid, latent curing polymerizable adhesives activated by heat and/or pressure also has been suggested. Schultz et al., U.S. Pat. No. 3,179,143 describes a metal fastener precoated with an adhesive, with one coactant separated from another by encapsulation in microscopic capsules which rupture under pressure when the fastener is tightened. Such adhesives, however, contemplate pressures generated by metal or metal abuttment surfaces when installed or activation through a separate heating step.
Anti-corrosion coatings for screws and bolts are also known. In addition, partially plastic coated waterproof screws have been proposed. See, for example, Sakayori et al. U.S. Pat. No. 4,788,022, issued Nov. 29, 1988. The present invention utilizes these and other techniques to address the various drawbacks of known roof fastener systems as described above.